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This species was first described by Linnaeus in his 1758 edition of Systema Naturae. Linnaeus' primary reference was his earlier Fauna Svecica, whose cumbersome pre-binomial name Parus capite nigro: vertice albo, dorso cinereo, pectore albo ("black-headed tit with white nape, ash-grey back, white breast") became the much simpler yet no less unequivocal Parus ater. This name – meaning "dusky-black tit" – was simply adopted from older ornithological textbooks, ultimately going back to Conrad Gessner's 1555 Historiae animalium. Linnaeus' description was essentially the slightly rephrased species name from Fauna Svecica: P[arus] capite nigro, dorso cinereo, occipite pectoreque albo. – "a black-headed tit, with ash-grey back, and white back of the head and breast." He gave no type locality except "Europe", but his original description refers to the population inhabiting Sweden (which is consequently included in the nominate subspecies today).[1]

Gessner also notes that the coal tit was known as Kohlmeiß in German – the literal equivalent of its English name, though in its modern orthographyKohlmeise it refers to the great tit (Parus major). That bird was in Gessner's day usually called Spiegelmeiß ("multicolored tit"[2]), Brandtmeiß ("burnt tit") or grosse Meiß ("great tit") in German. Kölmeyß was attested for P. major by William Turner, but Turner does not list P. ater at all, while Gessner notes that his hunters always used Kohlmeiß for the present species. However, this has since changed, and the modern German name of P. ater is Tannenmeise ("fir tit"), after a typical habitat. This name is attested (as Tannen-Maise) by Johann Leonhard Frisch in the early 18th century already, who furthermore records that P. ater was also called Kleine Kohl-Maise ("small coal tit") whereas Kohl-Maise referred unequivocally to P. major. Frisch collected his data in the Berlin region, where the German dialect was quite different from that spoken by Gessner's Alemannic sources 200 years earlier, and heavily influenced by Middle Low German – the language of the northern German sources of Turner. Regarding that, Tanne is derived from the Old Saxondanna, and thus had spread through the German dialect continuum from north to south. [3]

In addition, the same data suggest that this species is paraphyletic in regard to the closely related and parapatricspot-winged tit (P. melanolophus) from South Asia, which looks like a slightly crested, darker version of P. ater. Consequently, the spot-winged tit might have to be included in P. ater, or some coal tits could be considered a distinct species. As occasional hybridization has been recorded between the two, mtDNA alone (which is inherited only from the mother) is insufficient to determine whether hybrid gene flow or another trivial cause (such as incomplete lineage sorting) obfuscates the actual relationships, or whether taxonomic rearrangement is indeed required. With the range of these titmice encircling the Himalayas, without further study it cannot even be excluded that they constitute a ring species – with gene flow occurring in Nepal but not in Afghanistan –, as has been shown for other passerines in the same region.[4]

A number of coal tit subspecies are distinguished. The differences in coloration are quite pronounced in some of them, while their differences in size are more subtle. Coal tits from Asia follow Bergmann's rule, being larger in colder regions; those from further west, however, do not, as the birds from the uplands around the Mediterranean are larger than those from northern Europe. Across its range, tail length in relation to body length increases along a cline running from southwest to northeast.[5]

The British race P. a. britannicus has an olive hue to its brownish-grey back plumage, distinguishing it from the continental Europeannominate subspeciesP. a. ater and P. a. abietum[verification needed] in which the back is bluish grey without a hint of green or brown. The Irish race P. a. hibernicus is distinguished from britannicus by the pale sulphur-yellow cheeks, breast and belly. It also has a paler rump (due to light fringes of the uppertail coverts) and a larger bill than its relatives from Britain and the Continent.[6]

The North African race P. a. ledouci has yellow underparts and cheeks, and the CypriotP. a. cypriotes has a buff tinge to its upperparts, and deep buff underparts. Asian subspecies are generally rather dusky brownish except for the black-and-white head;[5] they include among others P. a. michalowskii of the Caucasus, P. a. phaeonotus of Iran, or the Himalayan coal tit[7]P. a. aemodius of southwestern China.

The coal tit is 10–11.5 cm in length, and has a distinctive large white nape spot on its black head. The head, throat and neck of the adult are glossy blue-black, setting off the off-white sides of the face (tinged grey to yellow depending on subspecies) and the brilliant white nape; the white tips of the wing coverts appear as two wingbars. The underparts are whitish shading through buff to rufous on the flanks. The bill is black, the legs lead-coloured, and irides dark brown.

The young birds are duller than the adults, lacking gloss on the black head, and with the white of nape and cheeks tinged with yellow.

While searching for food, coal tit flocks keep contact with incessant short dee or see-see calls. The species' song – if "song" it can be called – is a strident if-he, if-he, if-he, heard most frequently from January to June, but also in autumn. One variant of this song ends with a sharp ichi. North African birds also have a currr call similar to that of the European crested tit (Lophophanes cristatus) which is not found in Africa.

The coal tit is an all-year resident throughout almost all range, making only local movements in response to particularly severe weather; only the Siberian birds have a more regular migration. Very rarely, vagrants may cross longer distances; for example the nominate subspecies of continental Europe was recorded in Ireland once in 1960 and once before that, but apparently not since then.

Coal tits will form small flocks in winter with other tits. This species resembles other tits in acrobatic skill and restless activity, though it more frequently pitches on a trunk, and in little hops resembles a treecreeper (Certhia). Its food is similar to that of the others; it is keen on beechmast, picks out the seeds from fir (Abies) and larch (Larix) cones, and joins Carduelisredpolls and siskins in alders (Alnus) and birches (Betula). It will also visit gardens to feed on a variety of foods put out, particularly sunflower seeds.

A favourite nesting site is a hole in a rotting tree-stump, often low down, and the nest is deep within the hole; holes in the ground, burrows of mice or rabbits, chinks between the stones in walls, old nests of Pica magpies or other large birds, and squirrel dreys are also occupied. The materials, moss, hair and grass, are closely felted together, and rabbit fur or feathers added for lining. Seven to eleven red-spotted white eggs of the usual tit type are laid, usually in May; this species breeds usually once per year.

^Literally "mirror tit" (though its feathers are not iridescent), perhaps rather "wing-stripe tit", as in German ornithologySpiegel means a wing-stripe or -patch. The interpretation referring to its colorful plumage, though somewhat unusual, is the one given by Gesner however: a colorum pulchritudine quibus distinguitur – "for the beauty of its colors, which distinguish it"